block/pflash_cfi02: Implement intereleaved flash devices

It's common for multiple narrow flash chips to be hooked up in parallel
to support wider buses. For example, four 8-bit wide flash chips (x8)
may be combined in parallel to produce a 32-bit wide device. Similarly,
two 16-bit wide chips (x16) may be combined.

This commit introduces `device-width` and `max-device-width` properties,
similar to pflash_cfi01, with the following meanings:
- `width`: The width of the logical, qemu device (same as before);
- `device-width`: The width of an individual flash chip, defaulting to
  `width`; and
- `max-device-width`: The maximum width of an individual flash chip,
  defaulting to `device-width`.

Nothing needs to change to support reading such interleaved devices but
commands (e.g., erase and programming) must be sent to all devices at
the same time or else the various chips will be in different states.

For example, a 4-byte wide logical device can be composed of four x8/x16
devices in x8 mode. That is, each device supports both x8 or x16 and
they're being used in the byte, rather than word, mode. This
configuration would have `width=4`, `device-width=1`, and
`max-device-width=2`.

In addition to commands being sent to all devices, guest firmware
expects the status and CFI queries to be replicated for each device.
(The one exception to the response replication is that each device gets
to report its own status bit DQ7 while programming because its value
depends on the value being programmed which will usually differ for each
device.)

Testing is limited to 16-bit wide devices due to the current inability
to override the properties set by `pflash_cfi02_register`, but multiple
configurations are tested.

Stop using global_qtest. Instead, package the qtest variable inside the
FlashConfig structure.

Signed-off-by: Stephen Checkoway <stephen.checkoway@oberlin.edu>
Acked-by: Thomas Huth <thuth@redhat.com>